The present exemplary embodiments relate to the data processing arts. They find particular application in conjunction with printing or other image marking applications in which the print or marking resolution is less than the image rasterizing resolution. However, it is to be appreciated that the present exemplary embodiments are also amenable to other applications, such as image size reduction, data compression for data transmission or storage, anti-aliasing via jaggie reduction, and so forth.
Typically, print quality as perceived visually increases with increasing resolution in binary raster image processing. Digitizing artifacts such as “jaggie” edges, moiré patterns, and abrupt grayscale shifts are reduced at higher resolution. However, bandwidth increases with the square of the resolution, so that higher resolution images are more expensive to accommodate because of the higher bandwidth and memory requirements. Moreover, the cost of printers and other marking engines generally increases with increasing marking resolution capability.
Often, the rasterization of a document is performed at the same resolution as is supported by the marking engine. A higher resolution rasterization would generate higher quality binary. However, applying a conventional sub-sampling technique to lower the resolution would eliminate this quality advantage. It is therefore desirable to develop novel sub-sampling techniques that mitigate image degradation due to the sub-sampling.
In a technique known as half-bitting, the rasterization alternates between black and white states near the edge of a letter of text or a line of line art. Such half-bitting reduces jaggie edges and moiré patterns by more faithfully representing the boundary between the black and white regions. While half-bitting can be useful for text and line art, it does not improve, and indeed may even degrade, halftone images. Moreover, half-bitting that is designed to improve vertical and/or horizontal lines can cause problems when applied to slanted lines.
In a technique known as stoclustic halftoning, the halftone dither array is randomly or pseudorandomly varied to produce texture variation in the halftone image. Stoclustic halftoning can soften transitions between image regions exhibiting different gray levels. However, stoclustic halftoning methods employ large stored arrays of dithering patterns and typically reduce rasterization productivity. Moreover, stoclustic halftoning techniques are generally not applicable to text and line art. There are some other effective anti-alias rendering techniques that are effective on text and line art only. However, these techniques are typically computationally intensive. Moreover, a thorough anti-aliasing approach requires use of both techniques, as well as a costly segmentation scheme to distinguish between pictorials and text or line art.
Accordingly, there remains a need for improved sub-sampling apparatuses and methods which are generally applicable for improving text, line art, and halftone images, and have low computational complexity, and which are readily adapted to existing rasterization and printing systems, and which have other advantages over existing apparatuses and methods.